1. Field of the Invention
This invention relates to a group III nitride compound semiconductor device. More particularly, it relates to an improvement in an undercoat layer for a group III nitride compound semiconductor layer such as a GaN semiconductor layer.
The present application is based on Japanese Patent Applications No. Hei. 11-58128, 11-60206, 11-611155, 11-90833, and 11-235450, which are incorporated herein by reference.
2. Description of the Related Art
The fact that a (111) face of metal nitride exhibiting an NaCl structure as an undercoat layer is used as a substrate to obtain a group III nitride compound semiconductor layer, such as a GaN semiconductor layer, of a good crystal has been disclosed in Japanese Patent Publication No. Hei. 9-237938. That is, in Japanese Patent Publication No. Hei. 9-237938, metal nitride exhibiting an NaCl structure is used as a substrate so that a group III nitride compound semiconductor layer is grown on a (111) face of the substrate.
A substrate for a semiconductor device needs certain characteristics such as stiffness, impact resistance, etc. for keeping to maintain the function of the device. It is thought that the substrate needs a thickness of 100 μm or larger in order to keep the characteristic when the substrate is formed of metal nitride.
Metal nitride having such a thickness, however, has not been provided as a raw material of an industrial product used for production of a semiconductor.
Therefore, when an invention described in Japanese Patent Publication No. Hei. 9-237938 is to be carried out, the substrate of metal nitride must be produced personally (probably by a sputtering, or the like) with a great deal of labor.
Incidentally, it is known that, among group III nitride compound semiconductors, a GaN semiconductor can be used for a blue light-emitting device. In such a light-emitting device, sapphire is generally used as a substrate.
One of the problems to be solved in the sapphire substrate is as follows. That is, the sapphire substrate is transparent, so that light of the light emitting device to be originally taken out from an upper face of the device passes through the sapphire substrate. Hence, light emitted from the light-emitting device cannot be used effectively.
Moreover, the sapphire substrate is expensive.
Moreover, the sapphire substrate is an electrical insulator, so that it is necessary to form electrodes on one face side. Hence, the semiconductor layer must be etched partially, so that a bonding process twice as long is required correspondingly. Further, because n-type and p-type electrodes are formed on one face side, reduction of the device size is limited. In addition, there is a “charge-up” problem.
On the other hand, substituting an Si (silicon) substrate for the sapphire substrate may be thought of. According to the inventors' examination, it was, however, very difficult to grow a GaN semiconductor layer on the Si substrate. On cause of the difficulty is the difference in thermal expansibility between Si and the GaN semiconductor. The linear expansion coefficient of Si is 4.7×10−6/K whereas the linear expansion coefficient of GaN is 5.59×10−6/K. The former is smaller than the latter. Accordingly, if heating is performed when the GaN semiconductor layer is grown, the device is deformed so that the Si substrate is expanded while the GaN semiconductor layer side is contracted relatively. On this occasion, tensile stress is generated in the GaN semiconductor layer, so that there is a risk of occurrence of cracking as a result. Even in the case where cracking does not occur, distortion occurs in the lattice. Hence, the GaN semiconductor device cannot fulfill its original function.
FIG. 26 shows an example of a group III nitride compound semiconductor device with a group III metal nitride semiconductor layer grown on a sapphire substrate. In a semiconductor device 1, all of a p-type layer 6 and a light-emitting layer 5 and part of an n-type layer 4 are removed by means of etching. Further, an n-type electrode 9 is connected to a revealed portion of the n-type layer 4. Incidentally, in FIG. 26, the reference numerals 2, 3, 7 and 8 designate a substrate, a buffer layer, a light-transmissible electrode and a p-type electrode respectively.
The inventors have examined the light-emitting device configured as described above. As a result, problems to be solved have been found as follows.
The thickness of the substrate 2 is about 100 μm, and the thickness of each of layers 3 to 7 is about 5 μm. On the contrary, size of the lateral direction of the semiconductor device 1 is about 350 μm. When the LED is put on the light, the current must be flowed in the lateral direction of the n-type layer 4. As a result, the distance of the electricity path increases and resistance is increased unavoidably. Further, the thickness of an n-type layer portion 4A touching the light-emitting layer 5 is different from the thickness of an n-type layer portion 4B which forms an n-electrode 9. Hence, current concentration occurs in the boundary between the thick portion 4A and the thin portion 4B. As a result, the operating voltage of the device becomes high. There is also a problem that withstand electrostatic stress characteristic is worsened because of the current concentration. Moreover, when the aforementioned configuration is applied to a general electronic device such as a rectifier, a thyristor, a transistor, an FET, or the like, the operating voltage of the device becomes high. Hence, there is a further problem that the permissible current cannot be set to be large.